Diagnosis and treatment of many genetic diseases is hindered by variation in disease symptoms. The reasons for this variation are often unknown because systematic studies have not been undertaken. The goal of this Program Project is to elucidate mechanisms underlying normal phenotypic resilience and the instability that occurs when such mechanisms are lost in the disease state. We hypothesize that variation can result from failure of mechanisms that normally buffer against noise in developmental processes, for example stochastic variation in gene activities and cellular read-outs, and thus assure phenotypic stability in healthy children. We test this hypothesis in three Projects (Projects), using the zebrafish, a premiere model organism pioneered by this group at the University of Oregon. The projects take advantage of attributes of the zebrafish for developmental genetics analyses, including exquisite time-lapse microscopy of transgenically labeled fish to follow developmental events and perturbations in real time, assays of macromolecular complex formation, genome-wide analyses of cell signaling events, and assessment of environmental interactions that modulate host gene expression. These studies will elucidate the nature of events leading to variability in disease symptoms. Project1 focuses on variation In Fraser syndrome, a rare inherited disorder characterized by craniofacial and pharyngeal epithelial disruptions that show a remarkable degree of variation, both among affected individuals and on the left and right sides of the same individual. Proposed studies will explore how failure of epithelial-mesenchymal interactions results in differences in craniofacial skeletal development and will reveal genes responsible for the stability and resilience seen under normal conditions. Project2 tests a novel hypothesis for phenotypic variation with Usher syndrome, the most prevalent cause of hereditary deaf-blindness, hypothesizing that it results from disruption of complexes of Usher proteins that cause cellular stress that leads to stochastic cell death. Project3 investigates phenotypic variation associated with Hirschsprung disease, the leading cause of intestinal aganglionosis, exploring the hypothesis that the enteric nervous system regulates composition of intestinal bacterial communities and that altered communities contribute to disease progression by promoting inflammation and amplifying intestinal motility defects. Together with support of four Core Units, this Program Project will provide novel insights into three specific diseases and develop a new understanding of the mechanisms underlying disease variability that will promote better disease diagnosis and treatment.